In the manufacture of rubber articles, crude or raw rubber is compounded with various ingredients among which are sulfur and accelerators of vulcanization. The primary function of an accelerator or accelerator system is to increase the rate of the vulcanization process while allowing sufficient time to mix the accelerators into the rubber at an elevated temperature before vulcanization commences. This delay before the initiation of vulcanization is commonly referred to as scorch time.
The properties of a final rubber vulcanizate that are of importance include tensile strength, set, hysteresis, aging properties, reversion resistance and others. Other factors relating to the vulcanization which are of importance are the rate of cure, the cure time, the scorch behavior, the extent of cure and tear resistance. These physical properties can be altered either beneficially or detrimentally through the inclusion of chemicals or components that impact upon the rate and state of vulcanization.
Over the last 20 years, three broad classes of sulfur vulcanization system have been recognized for natural rubber. The systems are differentiated by the relative levels of sulfur and organic accelerator (Table I).
TABLE I ______________________________________ Sulfur Vulcanization Systems parts phr Sulfur Accelerator ______________________________________ Conventional 2-3.5 0.3-1 Semi-EV.sup.a 1-2 1-2.5 EV.sup.a 0.25-0.7 2.5-5 ______________________________________ .sup.a Efficient vulcanization.
At optimum cure, conventional systems give vulcanizates with mostly polysulfidic crosslinks and a high level of main-chain modification. Low sulfur and high accelerator levels give mainly monosulfidic crosslinks and much less chain modification; they are known as "efficient" vulcanization (EV) systems. Intermediate semi-EV systems afford a compromise between the high tensile strength, tear and fatigue properties coupled with susceptibility to reversion and oxidative aging of conventional systems and the high levels of resistance to reversion and aging, but lower fatigue, strength and wear properties of EV systems.
A wide range of accelerator combinations can be used within these systems and attention has focused mainly on sulfur:accelerator ratios to determine the choice for a particular application. However, there is evidence that the nature of the accelerator has an influence on the aging and reversion resistance of natural rubber vulcanizates. It has also been suggested that the use of sulfur donors to reduce the level of elemental sulfur can improve reversion and aging resistance.
An accelerator may provide too slow a cure rate when used alone. It may be speeded up by adding small amounts of another accelerator or a so-called activator. The accelerators which are most commonly activated are the thiazoles including thiazole sulfenamides. The accelerators most commonly used as activators are the thiuram sulfides, dithiocarbamates and the guanidines.
One of the most widely used accelerator systems or combinations for diene rubbers consists of a benzothiazole sulfenamide and a thiuram sulfide or dithiocarbamate. These are safe, non-scorchy and provide adequate rates of cure in a reasonable length of time and result in vulcanizates with good physical properties. Nevertheless, in commercial operations, for example, in the manufacture of automobile tires, it is always desirable to improve productivity and cost. One means for increasing productivity is to reduce vulcanization time without sacrificing quality or operating safety. In the past, numerous cure systems have been evaluated, some of which significantly increase the rate of cure, but at a cost of reduced processing safety, scorch or reversion resistance and these limitations have restricted their use.
U.S. Pat. No. 2,061,520 is concerned with cyanuryl derivatives of dithiocarbamic acids. This patent discusses the use of these derivatives as accelerators for the vulcanization of natural rubber or artificial rubber-like masses. Specifically, this patent is concerned with a process which comprises heating a cyanuryl halogenide with a sodium salt of a dialkyl dithiocarbamic acid at a temperature between about 30.degree. and 60.degree. C. in the presence of a solvent. This patent does not suggest or disclose the use of cyanuryl derivatives of dithiocarbamic acids in combination with morpholine polysulfides nor the beneficial properties that can be obtained therewith in the vulcanization of natural or synthetic elastomers.
U.S. Pat. No. 3,366,598 is concerned with a process for the vulcanization of rubbers containing fillers. Specifically, this patent discloses a process for vulcanizing a vulcanizable material selected from the group consisting of rubber and rubber-like elastomers through the use of a vulcanization system consisting of (1) an effective amount of sulfur and (2) an effective amount of a mercaptotriazine compound such as 2-dimethylamino-4,6-dimercapto-s-triazine. This patent does not suggest or disclose the use of dialkyl dithiocarbamyl substituted triazines in combination with a morpholine polysulfide, such as morpholine tetrasulfide, to achieve unexpected benefits in the vulcanization reaction nor the highly desirable physical properties of the final vulcanizate.
U.S. Pat. No. 3,844,970 relates to a vulcanization accelerator combination and its use in the vulcanization of natural or synthetic rubbers made from halogen-free dienes. This patent discloses vulcanization accelerators such as triazine substituted in the 2, 4 and 6 positions in combination with at least one thiazole accelerator and optionally at least one thiuram accelerator. This invention specifically relates to a mixture of special accelerators, such as 2-ethylamino-4,6-bis-(N-cyclohexyl-sulphenamido)-s-triazine and dibenzothiazyl disulfide and a thiuramic accelerator if desired such as tetraethyl thiuram disulfide. This patent does not suggest the use of specific dialkyl dithiocarbamic substituted triazines in combination with morpholine polysulfides.
U.S. Pat. No. 4,012,332 relates to the discovery of an accelerator composition for the vulcanization of diene elastomers comprising a benzylthiazole sulfenamide, a thiuram sulfide and copper 2-mercaptobenzothiazole.
U.S. Pat. No. 4,116,855 relates to stabilized accelerator compositions for rubber. More particularly, the invention relates to an accelerator composition comprising N,N'-[(thiocarbonyl)thio]dimorpholine and an effective stabilizing amount of an acrylic amide or ester.
U.S. Pat. No. 4,339,577 relates to vulcanizable rubber compositions inhibited from premature vulcanization through the use of a compound such as 2-(thioamino)-4,6-diamino-1,3,5-triazines. This patent relates to the discovery that (thioamino)-triazines are premature vulcanization inhibitors. Surprisingly, reversal of the respective positions of the sulfur and nitrogen atoms with respect to the triazine ring has a significant effect upon the vulcanization activity of the compounds. Triazines having aminothio substituents are accelerators whereas triazines having thioamino substituents are premature vulcanization inhibitors. Compounds such as 2-(methylthioamino)-4,6-di(t-butylamino)-1,3,5-triazine are illustrative of the inhibitors of the '577 invention.
One skilled in this art is aware that conventional cure systems (normally comprised of a high percent of polysulfidic crosslinks) will afford high tensile, tear and fatigue properties while EV systems (comprised of a high percent of mono and disulfidic crosslinks) afford high resistance to reversion and aging but at the expense of lower fatigue and strength (tensile and tear).
The novel and unobvious use of morpholine sulfides in combination with triazines have resulted in cures which not only give the crosslink types and physical properties of a conventional cure but also the retention properties of an EV vulcanization system.
None of the pior art alone or in combination suggests the use of a morpholine polysulfide in combination with a dithiocarbamyl triazine for the vulcanization of rubbers. In addition, the prior art fails to appreciate or disclose the beneficial curing characteristics and final vulcanizate properties that can be obtained through the use of materials such as morpholine tetrasulfide and 2,4-bis(dimethyldithiocarbamyl)-6-chloro-1,3,5-triazine.